The present invention relates to a power supply system composed of a DC power supplier in which a plurality of DC power supply units provided with a unit rated current value Iu are connected in parallel, capable of adjusting an unit current of each power supply unit in accordance with a target value, and a controller for providing the target value to the DC power supply units.
There has been known that an electric power is fed to a load of required load capacity Is with a power supply system composed of a DC power supplier in which a plurality of DC power supply units provided with a unit rated current value Iu are connected in parallel, and a controller for controlling the number of operating units. With this related power supply system, an electric current mxc2x7Iu given by the product between the number m of operating units selected by the operating unit number controller and the unit rated current value Iu is supplied to the load as a load current.
In the related power supply system as described above, for example, four DC power supply units are connected in parallel to supply the load current 4Iu to the load of required load capacity Is. If one DC power supply unit is stopped, remaining three DC power supply units are employed for feeding electric power, in which the electric current of 3Iu is supplied to the load. Namely, the electric current to be supplied is greatly decreased to three-fourth. With two DC power supply units, the electric current drops to 2Iu or half of the normal condition, and it is almost impossible to operate the load of required load capacity Is. In this way, the related power supply system in which the operating unit number is controlled can only adjust the feeding current to the load at every step of the unit rated current value of the DC power supply unit, resulting in a problem that the electric power conformable to the required load capacity can not be fed.
Providing more DC power supply units making up the DC power supplier, it is possible to prevent the feeding current from greatly decreasing, when the DC power supply unit during operation is stopped. Also, the required load capacity Is can be easily changed. However, if the number of DC power supply units is increased, there is a problem that the costs of the DC power supply system rise correspondingly. Briefly, there was a problem with the related power supply system that various required load capacities Is can not be dealt with. Further, there was the need for preparing the capacity according to the capacity of the entire power supply system for the AC input breaker, irrespective of the load capacity.
Further, when the commercial power supply fails, an engine generator may be utilized. In this case, when the power supply system is activated by the engine generator, an excessive electric current exceeding an output capacity of the engine generator may flow, bringing about the danger that the engine generator may knock to make the output power state worse, and in the worst case, to stop the engine generator.
To prevent this false state, the engine generator having a capacity greater than the required load power and capable of outputting an excessive current flowing into the power supply system is prepared, or the number of power supply units is limited to the number of units capable of supplying the minimum load current.
However, if the capacity of the engine generator was increased, the installation cost becomes higher, or if the number of units is limited, the minimum load current might not be supplied when the power supply unit is stopped during the operation, due to some reason, resulting in a problem that the reliability is lowered.
It is an object of the invention to provide a power supply system in which a load current conformable to a required load capacity can be supplied.
In order to achieve the above object, according to the present invention, there is provided a power supply system which includes a DC power supplier, a required load capacity provider, an operating condition detector and a target value calculator. The DC power supplier includes a plurality of DC power supply units connected in parallel each having a unit rated current value Iu and outputting a unit current in accordance with a target value. The required load capacity provider provides a capacity value Is which is required in the attached load. The operating condition detector detects the number, m, of DC power supply units which are operating. The target value calculator calculates a target value by dividing the capacity value Is with the number, m, of operating DC power supply units and provides the calculated target value Is/m to each operating DC power supply unit.
Preferably, the target value calculator provides the unit rated current value Iu when the calculated value Is/m exceeds the unit rated current value Iu.
In the above configurations, an electric current conformable to the required load capacity can be supplied to the load.
Preferably, the power supply system further includes an AC input breaker connected to an input side of the DC power supplier in parallel with the respective DC power source units. Here, a capacity of the AC input breaker is equivalent to the capacity value Is.
In this configuration, an AC input breaker having a smaller capacity than employed for the related power supply system can be used, whereby the costs of the power supply system can be lowered correspondingly.
Further, there is no need of limiting the number of power source units even when an engine generator is employed, whereby the power supply system can always operate in prepared number of units. Therefore, it is not required to increase the capacity of the engine generator while being in highly reliable state, whereby the installation costs can be suppressed.
According to the present invention, there is also provided a method of controlling an electric power supplied from a power supply system to a load. The method including providing a capacity value Is which is required in the load; detecting the number, m, of operating DC power supply units in the power supply system; calculating a target value by dividing the value Is with the value m; and providing the calculated target value Is/m to each DC power supply unit. The method also includes adjusting the current from each DC power supply unit to coincide with the calculated target value.
It is also preferable that the control method further includes determining whether the calculated target value Is/m exceeds a unit rated current value Iu of each DC power supply unit and providing the unit rated current value Iu as the target value when the calculated value Is/m exceeds the unit rated current value Iu.